The present invention relates to a heat transfer sheet and, more particularly, to a heat transfer sheet capable of recording an image representation excelling in the density of developed colors, clearness and various fastness properties, esp., storability.
Heretofore, various heat transfer techniques have been known in the art, including sublimation type transfer systems wherein a sublimable dye is carried on a substrate sheet such as paper to make a heat transfer sheet, which is then overlaid on an imageable or image-receiving material dyeable with a sublimable dye, for instance, a woven fabric made of polyester to apply heat energy in the form of a pattern from the back side of the heat transfer sheet, thereby transferring the sublimable dye onto the image-receiving material.
More recently, there have been proposed techniques for making various full-color images on paper or plastic films with the above sublimation type of heat transfer systems, in which thermal heads of printers are used as heating means to transfer three-, four- or more-color dots to image-receiving materials by quick heating, thereby reproducing or reconstructing full-color images of manuscripts with said multicolor dots.
Because of the coloring materials used being dyes, the thus formed image representations are very clear and because of being excellent in transparency, the obtained image representations are improved in the reproducibility and gradation or gray scale of neutral tints, are equivalent to those achieved by conventional offset or gravure printing, and are comparable in quality to full-color photographic images.
However, the most important problems with the above heat transfer systems are the color density, storability and resistance to discoloration/fading of the formed image representations.
In fast recording, heat energy is required to be applied within a time as short as fractions of a second. However, no image representations of sufficient color density can be obtained at all, since sublimable dyes and image-receiving materials are not well-heated within such short a time.
In order to cope with such fast recording, sublimable dyes excelling in sublimability have been developed. However, problems with such dyes of excellent sublimability are that after transfer, they migrate onto the image-receiving materials or bleed through with time, generally because of their low molecular weight. In consequence, the image representations become disfigured or blurred, or otherwise contaminate surrounding articles.
Even when a sublimable dye having a relatively high molecular weight is used to avoid such problems, no image representation of satisfactory color density can be obtained whatsoever, since its rate of sublimation is too slow for such fast recording as mentioned above